For indoor mosaics: I use Aleen’s tacky glue, it is thicker than regular white glue.
For outdoor mosaics: Use Weldbond glue for small verticle & round projects. Find it in Seattle at Hardwicks: 4214 Roosevelt, Seattle 98105 / 206-632-1203 or online at www.weldbond.com
For mosaics on cement, and mosaics in standing water, such as a bird bath or shower surround, I use thinset. Used since ancient times, thinset holds heavy tiles in place (prop very heavy tiles in place using a board) on a vertical surface. Premixed Thinset is easiest to use. It’s usually sold by the quart and gallon at your local hardware store.